It is common practice to provide pagers which emit an audible tone indicating that a particular recipient is to receive a message. Thereafter, the message is transmitted via voice communication to the recipient whether or not this recipient is ready to receive the message. As a result, the content of the message is sometimes lost when the recipient either cannot remember the content of the message or a pencil and paper is not immediately available to transcribe the message. This results in frustration of the recipient and the necessity of communicating with the originator of the message to obtain its content. In one broad aspect this invention alleviates the inconvenience of the prior art paging systems by providing storage and recall of the transmitted message through the use of a printed tape (hard copy unit).
As illustrated in U.S. Pat. No. 3,846,783 issued to Aspell et al. on Nov. 5, 1974, it is known to provide a pager with a hard copy printout utilizing a thermal print head. As discussed in this patent the thermal print head is advanced with the printing of each character. This presents a problem of synchronizing the print head advancement with the read out from the ASCII decoding logic. Not only must synchronization circuitry be supplied, but also the mechanical stability of the system must be such as to accomodate a moving print head or moving tape which is indexed with the printout of each character of the message. This not only requires complicated indexing of the print head relative to the printing medium, or vice versa, but also requires bulky drive apparatus which draws excessive current. As will be described, in accordance with one aspect of this invention, the subject invention utilizes a "printing-on-the-fly" technique which utilizes a fixed print head and a free running tape to eliminate the necessity of synchronism between the print head and the recording tape. This system obviously, therefore, eliminates the necessity of indexing.
"Printing-on-the-fly" refers to the continuous movement of the printing medium past a print head without indexing the head with respect to the printing medium, or vice versa. While printing-on-the-fly has been accomplished before with thermal print heads as illustrated in U.S. Pat. No. 3,777,116 issued to Riccardo Brescia and Lucio Montanari, its adaptation to a hand held pager is unique. One of the reasons for the uniqueness of this application is the problem of power drain from the batteries by the tape drive itself. Where power is plentiful, printing-on-the-fly is not a problem because the tape drive speed may be made substantially constant. However, in a battery powered operation, especially when the batteries are small, such as in pagers, battery voltages vary widely. In the past, in battery powered tape drives extra circuitry has been utilized to stabilize the speed of the motor in view of the start-up voltage transient as well as variation in the battery voltage with constant high drain. This has been accomplished most notably in the pocket cassette-type tape recorders which utilize magnetic tapes and audio signals. However, the "printing-on-the-fly" technique as described herein obviates the necessity for complicated and, in some instances, excessive power-draining motor drive control circuits because it is a finding of this invention that the speed of the tape drive motor may vary by as much as 50% and still produce a readily readable message. Hence, there need be no indexing of the thermal print head with respect to the paper and, more importantly, there need be no special control of the speed of the tape drive motor. This lack of synchronism and lack of criticality has been discovered and lies at the heart of providing an exceptionally simple hard copy, long life pager. Message samples with various tape drive rates are illustrated hereinafter to illustrate the range of speeds acceptable and thus the lack of necessity of tape speed control.
"Printing-on-the-fly", therefore, eliminates the necessity for indexing equipment which eliminates reliability problems, cost, and, in some instances, excessive power drain in battery powered units. The subject system is extremely reliable due to the lack of criticality between the tape movement and the printing speed.
It will be appreciated that the subject battery powered pager using the "print-on-the-fly" technique differs from the spacecraft teleprinter of U.S. Pat. No. 3,476,877 issued Nov. 4, 1969 to P. E. Perkins et al. in that there is no true addressing and thus no true paging function in the Perkins et al. system. More importantly, the Perkins et al. device indexes the tape with respect to the thermal print head, which approach is rejected in favor of the "print-on-the-fly" technique described herein. While satellites may be battery powered, power is not a problem due to the use of solar cells and thus the extra power drain and complexity of a stepping motor and drive circuitry of the Perkins et al. system can be tolerated, especially where the satellite is considerably larger than a personally portable pager. For personally portable pagers however, space and power are at a severe premium. Moreover pagers are subjected to more physical abuse than satellite circuitry. Thus the reductions of power drain and mechanical complexity afforded by "printing-on-the-fly" offer unusual advantages to a pager system. While the "print-on-the-fly" pager is the subject of this patent, several entire paging systems will be described hereinafter to point up the types of systems to which the subject invention is applicable and to contrast it with other systems.
It is therefore an object of this invention to provide a pager with a printing system in which there is continuous relative movement between the printing head and the printing medium without significant distortion of the printed characters.